1. Field of the Invention
This invention relates to a class of vibratory rotation sensors in which the vibrating members are thin-walled axisymmetric hemispherical shells, and more specifically pertains to the control electronics for such a sensor.
2. Description of Related Art
A prior art vibratory rotation sensor 10 is illustrated in FIG. 1, as having an outer member 12, a hemispherical resonator 14, and an inner member 16, all of which are made of fused quartz and are joined together with indium. This particular type of vibratory rotation sensor which has a vibrating member 14 that is a thin-walled axisymmetric hemispherical shell is known as a hemispherical resonator gyro (HRG).
The inertially sensitive element in the HRG is the hemispherical resonator 14, usually a thin-walled 5.8 cm diameter bell-shaped object positioned between the outer member 12 and the inner member 16 and supported between the inner and outer members by a stem 26.
The thin-walled axisymmetric hemispherical shell 14 oscillates in one of its lower flexing modes which takes the form of a standing wave. The standing wave which exists around the rim of the shell resonator 14, is shown in the two extremes 26 and 28 of its oscillatory deformation in FIGS. 2 and 3.
The elliptical standing wave contains four antinodes and four nodes. The antinodes and nodes are separated from one another by 45.degree.. The rotation sensitivity of the standing wave results from the fact that each mass element of the shell undergoing oscillation acts much like a Foucault pendulum attempting to keep the direction of its linear momentum fixed in inertial space when the shell rotates about its axis. The resulting Coriolis forces, the product of the shell's vibratory motion and the inertial input rate causes the standing wave to precess with respect to the shell. The ratio of the standing wave precession angle to the inertial input rotation angle is known as the angular gain of the gyro.
In operation, forces are required to control the standing wave on the shell resonator 14. These forces are quasi-electrostatic in nature. In the case of the HRG in FIG. 1, a number of electrodes 22 are metalized on the inside surface 20 of the outer housing 12 which is concentric with the hemispherical shell resonator 14. The outer surface of the shell resonator 14 is metalized so that when the device is assembled, the electrodes in the outer housing 12 together with the surface of the resonator they face form a series of forcing electrostatic capacitors. Voltage is applied to the appropriate combinations of these electrodes to control the amplitude of the standing wave and to suppress unwanted quadrature effects.
Rotation of the HRG 10 about an axis normal to the plane of the rim 34 of shell resonator 14 causes the standing wave to rotate in the opposite direction with respect to the HRG by an angle proportional to the angle of rotation of the HRG 10. Thus, by measuring the angle of rotation of the standing wave with respect to the HRG 10, one can determine the angle of rotation of the HRG 10.
The vibrational mode of the shell resonator 14 is excited by placing a DC bias voltage on the resonator and an AC voltage on the forcing electrodes 20. The frequency of the AC voltage is usually twice the resonant frequency of the hemispherical shell resonator 14.
Readout signals from the HRG containing information about the amplitude and location of the standing wave on the shell resonator 14 are also obtained capacitively. Capacitor readout electrodes 24 are formed by metalized interior surface 30 of the shell resonator 14 and a plurality of electrodes 24 which are located on an inner concentric housing held in close proximity to the inner metalized shell resonator 14. Because of the shell's oscillating deformation the capacitance of each of the electrodes 24 is modulated at the resonator flexing frequency. Electronic readout circuits measure these capacitance changes and hence the location and amplitude of the standing wave is determined.
Additional and more specific details of vibratory rotation sensors can be found in U.S. Pat. No. 4,951,508 issued to Loper, Jr., et al. on Aug. 28, 1990, the entire disclosure thereof being incorporated herein.
Eight electrodes 24 (FIG. 1) are usually metalized on the surface of the pick-off assembly. These eight electrodes schematically illustrated as 30 and 32 in FIGS. 2 and 3 are connected together to form two sets of four. Each group 30 and 32 of four electrodes measures the amplitude of the standing wave pattern over the electrodes. Output from the group one electrodes 30, pick-off axis No. 1 is: EQU PO1 (pick-off axis No. 1)=A cos. (2 PA)
Output from the group 2 electrodes 32, pick-off axis No. 2 is: EQU PO2 (pick-off axis No. 2)=A Sin (2 PA)
The pattern angle (PA) is computed from EQU .sup.PO2 /.sub.PO1 =Tan. (2 PA).
There are two methods of operating the HRG. The force to rebalance method (FTR) and the whole angle method (WA). In the force to rebalance method, an electrostatic force is applied to the resonator to lock the pattern angle around zero. When the pattern angle is close to zero the output of the pick-off axis EQU PO1=A cos. (2 PA)=A and PO2=A sin. (2 PA)=A (2 PA).
Because PO2 is a small signal, a large gain may be applied to the signal to increase resolution and sensitivity of the HRG.
In the whole angle method of operation applied inertial rate causes the pattern 26 to move relative to the pickoff axis 34 to a new position 36 (FIG. 2). This difference 38 is the pattern angle PA. The output signals PO1 and PO2 from the two groups of electrodes must be scaled to handle larger signals. Measuring the standing wave components along the pickoffs and then taking the arc tangent of the ratio of their amplitudes provides a measure of the pattern angle PA 38.
In the whole angle tracking mode of operation shown in FIG. 4, the digital signal processing control 52 controls the AC excitation voltages 50 so that the readout signals from the resonator 41 are proportional to the motion at the standing wave nodes and antinodes.
Further detail about whole angle tracking mode of operation can be obtained by reference to co-pending patent application U.S. Ser. No. 802,009 titled Vibratory Rotation Sensor With Whole-Angle Tracking by Matthews, Varty, Li and Lynch, U.S. Pat. No. 5,801,310, granted Sep. 1, 1998.
A circuit for accomplishing this operation is illustrated in FIG. 4 as having the HRG 40 with its resonator 41 and Group 1 and Group 2 electrodes 43. The standing wave components in this case are measured from the resonator 41 by way of an AC buffer 42. The standing way components are processed by an axis 1 processor 44 and an axis 2 processor 46. The resulting signals are supplied to an analog to digital converter (ADC) 48. The digital form of a resultant signal is supplied to digital signal processing control 52 which provides the pattern angle output on line 54 and a feedback excitation signal 50 to the excitation electrodes 43. A computer-generated excitation angle is servoed to the pattern angle. As a result, the signal can be amplified with very high gain to obtain a low-noise high accuracy readout signal without sacrificing excellent scale factor performance over a wide dynamic field.
When operated in the whole angle mode, the HRG possesses a high dynamic rate capability. However, many precision pointing and tracking control applications require low noise. The digitization of the whole angle mode readout introduces significant angle quantization noise; for example, approximately four arc seconds for a 16-bit ADC. In precision pointing applications, the quantization noise present must be reduced by a factor of greater than 1,000. This requires an analog to digital converter greater than 26 bits. Such analog to digital converters are presently not practical.
The whole angle tracking mode of operation as discussed above reduces the analog to digital converter bit requirement by forming a small error signal that is quantizable by a 12 to 16 bit analog to digital converter. In the whole angle tracking mode, high dynamic rate, low noise angle readout is achieved by slewing a set of electrically computed readout excitation signals. These signals, however, require a digital to analog converter (DAC) which again produces large angle quantization noise which must be corrected.